A premium threaded connection for oil country tubular goods (OCTG) has a metal-to-metal seal portion to ensure its gas tightness under high pressure and high temperature. The metal surfaces are coated with grease and then integrated by rotary sliding. Despite several studies, the gas tightness mechanism of the metal-to-metal seal has not yet been clarified. In this study, as the first step to clarify the mechanism, we evaluated how grease affects the gas tightness by fundamental tests considering rotary sliding and grease degradation due to high temperature. Our results showed that grease is mandatory for the gas tightness of the metal-to-metal seals and the gas tightness is affected by both types of base oils and solid additives of grease. The seals coated with the grease composed of mineral oil had better gas tightness than those coated with that composed of plant oil, even after degradation due to high temperature. In addition, grease containing soft metal particles had better gas tightness than grease containing hard inorganic particles because the extended soft metal particles adhering to the rough parts of the seal surfaces reduced the size of the leakage paths.

The Oil & Gas industry's increased activity in high pressure and high temperature developments requires effective solutions to meet the new challenges placed on threaded connections for Oil Country Tubular Goods (OCTG) to withstand the extreme loads imposed by these applications, such as combined external pressure and compression. Therefore, OCTG threaded connections have to withstand extremely high levels of combined loadings while maintaining structural and sealing integrity for tubing and casing strings. As of today, very few of the existing casing connections can comply with the severest protocol of ISO 13679, CAL-IV, within the full pipe body envelope; ISO13679 is the international standard detailing the testing procedures for casing and tubing connections. To meet these requirements, a new high-performance threaded and coupled casing premium connection was developed. This development was conducted by iterating Finite Element Analysis (FEA) results and prototype physical testing. As a first step, an innovative sealing technology consisting of a pioneering mid pin nose sealing mechanism with extended lip combined with a stabilizing feature was developed for a specific Outside Diameter (OD) size. This concept was then extended to a product line, and the connection geometry was optimized by FEA leading to a design freeze for OD's between 9-5/8" to 13-3/8" covering commonly used industry weights. As recommended by the ISO13679 standard, all of the designed sizes/weights were physically tested based on a polygon approach, that is, all of product line corners defined by sizes and wall thickness were verified by physical tests while sizes/weights falling within the polygon were verified by FEA. Ease of usage under adverse conditions of alignment was a major feature retained throughout the design phases of this new premium connection. On one hand, the anti-galling and the stabbing performances were optimized to fit automated rig handling systems thanks to a new thread profile. On the other hand, the seal geometry and its position were designed to protect the connection from damages due to rough handling. These features have been validated through extensive Make & Break tests and rig trials. Through this development, an innovative high-performance premium threaded connection, named VAM® 21, compliant with ISO 13679 CAL-IV protocol within the full pipe body VME envelope and having excellent handling and running performances is now available as a product line. Introduction With the increase development of HPHT and offshore deepwater wells in recent years, the performance requirements of OCTG and threaded connections, particularly compression loads and external pressure integrity are escalating. Although ISO 13679 is the worldwide standard for physical qualification testing of OCTG threaded connections, there has been so far very few high-performance premium threaded connections complying with this testing procedure, especially CAL-IV level protocol within the full pipe body envelope. This has prompted the need for a reliable premium threaded connection which is able to be assembled quickly and safely under adverse conditions of alignment often encountered on offshore automatic handling and running systems. To achieve trouble free make up performances as well as retaining sealing integrity within the full pipe body envelope, application of innovative technologies to premium threaded connections was needed.

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